“Endeavour”

All the coverage of the Space Shuttle Endeavour’s ongoing cross-country farewell tour made me wonder, naturally, about the ou spelling in its final syllable. It turns out it was named–u and all–after the first ship commanded by the eighteenth-century English explorer James Cook. It’s not a natural spelling for us Yanks, hence this mistake in a sign some well-meaning NASA folk constructed to cheer on a 2005 launch:

Despite the shuttle’s fame, the u-less spelling (indicated by the red line on this Ngram chart, showing use of both spellings in American English between 1800 and 2008) remains a strong preference on these shores, as it has been since 1850:

Not sure really. I would say “on the sign” too but in the post (above) “in” is used – hence my question. I imagine that a sign may be considered a container in that it might comprise symbols and words so “in” could be correct too. No biggy.

# :
This symbol is, or was used, at least before I retired, in medicine to indicate a bone fracture. My father, an engineer, used the symbol to indicate “Pounds per square inch” for pressure. And my musical friends use it to indicate a half tone increase (I think)
I was surprised many years ago, to see its use in North America where its use clearly meant “number” as an alternative to “No”..
Anyway. I believe all the NASA shuttles were named after famous ships used by intrepid explorers. Endeavour was a British ship as stated above.